Fabrication and use of polished silicon micro-mirrors

ABSTRACT

A method of fabricating silicon micro-mirrors includes etching from opposite sides of a silicon wafer with a polished surface on at least one of the opposite sides, to form silicon bars each having a parallelogram-shaped cross-section and including a portion of the polished surface. At least one of the silicon bars is mounted on a mounting surface. The polished surface of the silicon bar may be used to reflect optical signals.

BACKGROUND

The disclosure relates to the fabrication and use of polished silicon(Si) micro-mirrors.

Micro-mirrors may be used, for example, in optical sub-assemblies.Examples of such applications includes Transmitter Optical Sub-Assembly(TOSA) devices in which an edge emitter laser is mounted on a platformand the optical beam, parallel to the substrate surface, needs to beredirected by about ninety degrees. The lateral dimensions for suchmirrors can be in the range of 500 by 500 microns.

SUMMARY

The present disclosure describes a cost-effective method for fabricatingmicro-mirrors in high-volume using standard angle cut Si wafers as theraw material in a way that ensures high optical quality of the mirrorsurface.

In one aspect, a method of fabricating silicon micro-mirrors includesetching from opposite sides of a silicon wafer having a polished surfaceon at least one of the opposite sides, to form silicon bars each havinga parallelogram-shaped cross-section and including a portion of thepolished surface. One or more of the silicon bars are mounted on amounting surface. The polished surface of the silicon bar may be used toreflect optical signals.

In various implementations, one or more of the following features may bepresent. For example, the etching may include performing an anisotropicetch. Thus, in some implementations, the etching may include using KOHas an etchant and may include simultaneously etching from the oppositesides of the silicon wafer. In some implementations, the etching mayinclude simultaneously etching grooves in opposite sides of the wafer,wherein each of a pair of grooves in opposite sides of the wafer areslightly offset from one another so that, as the etching proceeds, thetwo grooves are merged together to form sides of the silicon bars. Insome implementations, the etching may include etching V-grooves withtheir longitudinal axis parallel to the wafer's<110>planes. The polishedsurface of the wafer may be parallel, for example, to thewafer's<100>planes. The etching may expose a surface in the wafer, andthe silicon bar may be mounted with the exposed surface facing themounting surface. In other implementations, different etching techniquesmay be used.

Other features and advantages will be readily apparent from thefollowing detailed description, the accompanying drawings and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a silicon mirror on a sub-mountaccording to the invention.

FIG. 2 illustrates a silicon wafer for fabricating the silicon mirrors.

FIG. 3 illustrates the silicon wafer with an etch mask.

FIG. 4 illustrates the silicon wafer after etching v-grooves from bothsides of the wafer.

FIG. 5 illustrates the wafer structure after removal of the etch mask.

FIG. 6 illustrates a side view of the mirror as it is oriented on thewafer.

DETAILED DESCRIPTION

As shown in FIG. 1, a discrete silicon micro-mirror 10 is mounted on anoptical sub-mount 12. In some implementations, the area occupied by themirror may be about 500 microns (μm) by 500 μm, with a thickness ofabout 300 μm. Different dimensions may be appropriate for otherimplementations. The mirror 10 includes an inclined polished surface 14which may be coated, for example, with aluminum (Al) or gold (Au). Thebottom surface 16 of the mirror may be coated, for example, with AuSnfor attachment to a die (e.g., the optical sub-mount 12) through areflow process. The upper surface 18 of the mirror also may be coated,for example, with Al or Au. In the illustrated example, the polishedsurface 14 of the mirror 10 forms an angle of about 45° with respect tothe plane of the optical sub-mount 12. However, in some implementations,the polished surface may form a different angle with the substrate onwhich it is mounted.

The following paragraphs describe a process for fabricating amicro-mirror such as the one illustrated in FIG. 1.

The fabrication process is based on a anisothropic wet etch of a siliconwafer using, for example, potassium hydroxide (KOH) as the etchant.During etching, the<111>planes of the Si crystal are revealed due to thelow etching rate in the direction perpendicular to those planes.

In one embodiment, the technique uses a Si wafer 20 (FIG. 2), which iscut such that the wafer's top surface will form a specified angle withthose planes. In this case, the wafer's top surface is parallel tothe<100>planes for standard wafers. For example, to achieve an angle of45° or 64.4°, the angle between the wafer's top surface andthe<100>crystal planes should be approximately 9.7°.

At least one of the two sides of the wafer (i.e., the upper or lowersurfaces) is polished to serve as the optical reflecting surface of themirror. In the example of FIG. 2, the wafer's upper surface 22 ispolished.

V-grooves, with their longitudinal axis parallel to thewafer's<110>planes, are etched through the wafer from both sides of thewafer 20. As shown in FIG. 3, an oxide or nitrogen-oxide pattern 24,which is provided on both surfaces of the silicon wafer 20, may be usedas the etching mask. The v-grooves 28A, 28B may be etched simultaneouslyfrom both sides of the wafer and may be slightly offset from one anotherso that, as the etching proceeds, two grooves etched from opposite sidesof the wafer are merged together such that the remaining silicon bars 26have a cross-section in the form of a parallelogram (see FIG. 4). Asmentioned above, KOH may be used as the etchant. In someimplementations, other etchants may be used. The angles of theparallelogram-shaped bars 26 are controlled by the crystal orientationof the wafer.

The mask pattern 24 may be stripped from the upper and lower surfaces ofthe wafer (FIG. 5). The wafer may then be diced to form individualsilicon mirrors, in which the polished upper surface 22 serves as thereflective surface 14 (FIG. 6). Prior to attaching the mirror to theoptical sub-mount, the surfaces of the mirror may be coated with Al, Au,AlSi, or another coating as appropriate. In addition, glue or solder maybe deposited on the back-side 16 to enable attachment to the opticalsub-mount. The mirror 10 is rotated, for example, by 45° or 135° fromits orientation on the wafer (FIG. 6) prior to being attached to thesub-mount 12 (FIG. 1). The angle θ between the polished surface of thesilicon bar and the mounting surface is around 135° (i.e., an obtuseangle).

The micro-mirrors may be used in various applications including ascomponents in optical sub-assemblies. Examples of such applicationsinclude Transmitter Optical Sub-Assembly (TOSA) devices in which an edgeemitter laser is mounted on a platform and the optical beam, parallel tothe substrate surface, needs to be redirected by about ninety degrees.

Other implementations are within the scope of the claims.

1. A method comprising: etching from opposite sides of a silicon waferhaving a polished surface on at least one of the opposite sides, whereinetching the wafer forms silicon bars each having a parallelogram-shapedcross-section and including a portion of the polished surface; andmounting at least one of the silicon bars on a mounting surface.
 2. Themethod of claim 1 wherein the etching includes performing an anisotropicetch.
 3. The method of claim 2 wherein the etching includes using KOH asan etchant.
 4. The method of claim 2 wherein the etching includessimultaneously etching from the opposite sides of the silicon wafer. 5.The method of claim 2 wherein the polished surface of the wafer isparallel to the wafer's<100>planes.
 6. The method of claim 1 wherein theetching includes simultaneously etching grooves in opposite sides of thewafer, wherein each of a pair of grooves in opposite sides of the waferare slightly offset from one another so that, as the etching proceeds,the two grooves are merged together to form sides of the silicon bars.7. The method of claim 1 wherein the etching includes etching V-grooveswith their longitudinal axis parallel to the wafer's<110>planes.
 8. Themethod of claim 1 wherein the angle between the polished surface of thesilicon bar and the mounting surface is between 90° and 180°.
 9. Themethod of claim 1 wherein the angle between the polished surface of thesilicon bar and the mounting surface is approximately 135°.
 10. Themethod of claim 1 wherein the etching exposes a surface in the wafer,and wherein the at least one silicon bar is mounted with the exposedsurface facing the mounting surface.
 11. The method of claim 1 includingseparating the silicon bars before mounting at least one of the siliconbars on the mounting surface.
 12. The method of claim 1 including usingthe polished surface of the silicon bar mounted on the mounting surfaceto reflect an optical signal.
 13. The method of claim 1 including usingthe silicon bar mounted on the mounting surface as a micro-mirror. 14.The method of claim 1 including incorporating the silicon bar into anoptical sub-assembly.
 15. The method of claim 1 including incorporatingthe silicon bar into a Transmitter Optical Sub-Assembly device.
 16. Themethod of claim 1 including incorporating the silicon bar into anassembly having an edge emitter laser for emitting an optical beamtoward a polished surface of the silicon bar, wherein the optical beamis redirected by about ninety degrees upon reflection by the polishedsurface of the silicon bar.